This is something I've debated on ad nausium my self- but this is the conclusion I came to.

I have my long sleeve base layer first, if it gets colder I put on the vest, AND my rain shell and am good until about 30 deg F, or a little lower. after that I need my down jacket (closer to 15 deg F, or a wind chill of the same)- which I always have just in case- and it doubles as my pillow when in a stuff sack...
Their is also nothing worse than being at the campsite without a long sleeve insulating unit if you need it, and that is for 4 season- even summer can have wind chills in the mountains (and I live in Georgia) so I use my GoLite Cayenne, and take it on every trip as a pillow if nothing else.